The Entity-Relationship Model

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The Entity-Relationship Model

Instructor: Mohamed Eltabakh meltabakh@cs.wpi.edu

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Database Design StagesApplication

Requirements

ConceptualDesign

Logical Design

Physical Design

Conceptual Schema

Logical Schema

Physical Schema

ER Model

Relational Model

Creating tablesin a DBMS

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Conceptual Design What is Conceptual Design?

Concise representation of the DB application requirements

Why Conceptual Design ? It helps to understand application requirements better It helps to communicate our understanding of application It helps to come up with a ‘good’ design

Example: Banking System There are customers, each customer has some attributes:

customer-id, customer-name, customer-street, customer-city. The bank offers loans to customers, where each loan has a

loan-number, and the amount of the loan A customer can borrow multiple loans from the bank, but each

loan can be borrowed by only one customer.

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This big story is captured in a better way using an ER model

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Conceptual Design (Cont’d) Conceptual Models

ER (Entity-Relationship) Model, Our focus UML (Unified Modeling Language), ORM (Object Role Modeling), etc.

ER Model Structures: Entities and Relationships Constraints

An ER schema is represented using Entity-Relationship Diagram (ERD)

Modeling A database can be modeled as:

Collection of entities, Relationship among entities.

An entity is an object that exists and is distinguishable from other objects. Example: a person, company, event, bank account, store, etc. Entities have attributes

Example: a person has name, address, SSN, …

An entity set is a set of entities of the same type that share the same properties. Example: set of all persons, companies, trees, holidays

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ER: Entity Sets and Attributes Entity set : Represent a set of “objects” Attribute: property of an entity, has a domain In ER diagram

Entity set rectangle Attribute Oval.

Student

sNamesNumber

sAge

Entity Set Studentwith attributes (sNumber, sName, sAge)

Example: University SystemWhat entity sets come to your mind…

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Student

ProfessorBuilding

Employee

Course

Classroom

Attributes Types Primitive attributes: Attribute stores a single value (Number,

String, Boolean, Date, …)

Composite attributes: An attribute can be divided into sub-attributes (each is primitive)

Multi-values attributes: Attribute with many values

Derived attributes: Attributes computed from others

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Student

sNamesNumber

sAge

These are primitive attributes

DoB

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Complex AttributesComposite Attribute: address Multivalued Attribute: major

major

sta testree t

address

city

Derived Attribute: Age

Age

Student entity setwith all its attributes

Student

sNam esN um ber

sAge

m ajor

sta testree t

address

city

Age

DoB

Expand Your Design: University System

Add attributes for each entity set

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Student

sN am esNum ber

sA ge

m ajor

statestreet

address

city

Student

Professor

Building

Employee

Course

Classroom

Name Address

Establishing Year

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Relationship Types Relationship: Association (connection) between entities sets Relationship Type: Class of relationships Representation: Use a diamond shape

Student

sNumber

sName

Course

cNumber

title

HasTaken

Relationship type HasTaken to represent Courses taken by Students

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Relationships with Attributes

Student

sNumber

sName

Course

cNumber

title

HasTaken

project

Relationship HasTaken has an attribute project which is theproject the Student did for the Course

Student

sNumber

sName

Course

cNumber

title

HasTaken

A relationship may have attributes These attributes do not belong to any of the connected entities. But

they belong the relationship

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Example I (Simple Application)

Suppliers have a name, and location.Products have a name, price, and number.Consumers have a name, and location.

Suppliers supply products on certain dates, while consumers buy products of certain quantity

How would you model this application? a) What are the entities and attributes? b) What are the relationships?

Modeling of Example I

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supplier

sName

sLoc

consumer

cName

cLoc

product

pNumber sName sPrice

supplies buys

date quantity

This ER captures exactly what is written in text>> It is wrong to add “date” or “quantity” to one of the entity sets

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Example II (More Complicated)

Suppliers have a name, and location.Products have a name, and number.Consumers have a name, and location.

Some Suppliers have established contracts to supply a certain Product to a particular Consumer for specially negotiated price at a given quantity.

How would you model this application?

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Modeling of Example II

Supplier

sName

sLoc

Consumer

cName

cLoc

Supply

price

Product

pName pNumber

qty

Model the relationship Supplier supplies Products to Consumers

Ternary relationship(three-way)

Binary vs. Multi-way Relationships

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supplier

sName

sLoc

consumer

cName

cLoc

product

pNumber sName

supplies buys

price quantity

Binary relationship

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ER Model so far Entities and entity sets

Relationships Binary, ternary, multi-way

Attributes For entity sets and relationship types Simple, composite, multi-valued, derived

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Entities with Different Attribute Types (Recap)

Composite Attribute: address

Multivalued Attribute: major

major

sta testree t

address

city

Derived Attribute: Age

Age

Student entity typewith all its attributes

Student

sNam esN um ber

sAge

m ajor

sta testree t

address

city

Age

DoB

Primitive Attribute: sNumber

sNumber

Binary Relationships (Recap)

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supplier

sName

sLoc

consumer

cName

cLoc

product

pNumber sName sPrice

supplies buys

date quantity

Attributes can be attached to Entity Sets or Relationships

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Multi-Way Relationships (Recap)

Supplier

sName

sLoc

Consumer

cName

cLoc

Supply

price

Product

pName pNumber

qty

Model the relationship Supplier supplies Products to Consumers

Ternary relationship(three-way)

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Recursive Relationship Types and Roles

Refer to the same entity set in the relationship

Recursive relationship type : Part-Subpart

Roles: There are Parts that play the role of superPartThere are Parts that play the role of subPart

Contains

Part

pName pNumber

subPartsuperPart

quantity

If two entities in the same entity set have a relationship Recursive relationship

Recursive Relationships: Another Example

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Employees & Managers

Employee

ID

Name

Supervisesupervisor

supervised

What about an Exercise: University System

Lets enrich this initial design…

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Student

sN am esNum ber

sA ge

m ajor

statestreet

address

city

Student

Professor

Building

Employee

Course

Classroom

Name Address

Establishing Year